To regulate the quality of air in occupied spaces it is necessary to insure that enough fresh air in introduced into the spaces to keep a comfortable and healthful atmosphere. For example, in office buildings air is circulated and recirculated through heating and cooling systems. Ultimately contaminants build up in the air and it becomes unhealthful to breathe as well as having a general feeling of staleness. Respiration introduces water vapor and carbon dioxide into the air and occupants of rooms cause other such things as perfume, cosmetics, perspiration and solvents to enter the atmosphere and contribute to the sense of air staleness.
Government regulations frequently require an amount of fresh air to be introduced into a recirculating air system for each cycle of recirculation. This approach to maintain air fresh is inadequate. It is approximate, it is difficult to effect because the quantity of recirculating air and the quantity of fresh air are difficult to monitor and because it does not take into account differences in population density, for example between an executive office and a crowded conference room.
Although it is difficult to monitor each of the contaminants that contribute to air staleness, one good measure of population density in an enclosed room that correlates closely with all of those contaminants is the carbon dioxide level of the air. It is a good indication of population because it is generated by respiration. Carbon dioxide level also correlates closely with carbon monoxide level in environments where internal combustion engines are used in enclosed spaces, for example in a garage or an underground mine.
In some industries it is desirable to monitor other gases. Chemical plants may monitor volatile material such as benzene and mines must monitor the methane level of the atmosphere both to maintain a healthy atmosphere and to avoid explosions.
Devices useful for monitoring the air desirably are inexpensive and small enough to be portable.
Many devices presently exist to monitor air for the presence for contaminant gases. Many of these devices function on the basis of the ability of contaminant gases to absorb infrared radiation. These devices enclose a column of gas to be monitored and place an infrared source at one end of the column and an infrared detector at the other and then introduce the gas to be monitored into the column. Infrared monitoring is desirable because specific narrow frequency bands of infrared radiation are very strongly absorbed by specific gases so the presence of those gases can be detected by measuring the absorption of infrared energy in those narrow bands. For example, carbon dioxide very strongly absorbs that portion of the infrared spectrum between 4.2 and 4.4 microns in wave length while methane strongly absorbs the portion between 3.1 and 3.6 microns in the wave length.
In many devices the quantity of a contaminant gas is measured by comparing the absorption in a column of gas known to contain none of the contaminant with the absorption of a column of the gas being analyzed. The gas being analyzed is pumped through a tube having an infrared source at one end and a detector at the other with appropriate optical filters between the source and the detector to pass only the narrow band most strongly absorbed by the contaminant for which the analysis is performed. To create an alternating current effect which is desirable from a signal processing standpoint, a chopper is positioned between the infrared source and the detector. After suitable calibration of the instruments, the difference between the amounts of infrared absorbed in the reference column and in the analysis column can be correlated to the concentration of the contaminant in the analysis column. Devices of this type are disclosed in U.S. Pat. Nos. 3,529,152 and 3,790,797.
The devices described above require moving choppers which operate in a free air optical path between the source of infrared and the gas column. This free air path may contribute to inaccuracies in the analysis because there is no control over a contaminant level in that path. The problem is particularly acute if sampling of the air to be monitored is remote from the position of the infrared source. The mechanical chopper also requires moving parts which are susceptible to breakage, which are noisy, which require maintenance, and which must be made explosion proof if used, for example, to monitor the methane level in a mine. In addition, air passed through the columns must be pumped and filtered so dust does not affect the absorption of infrared and such systems require moving parts to pump the air and filter maintenance to insure that it is clean and flowing at a uniform rate.